Resilient and flexible, linear and non-linear carbonaceous fibers are now well known in the art. Representative of non-linear carbonaceous fibers is U.S. Pat. No. 4,837,076, issued Jun. 6, 1989 to McCullough et al. The fibers are produced by melt or wet spinning fibers from a thermoplastic polymeric composition and then stabilizing the fibers by treating them in an oxygen containing atmosphere and at an elevated temperature for a predetermined period of time. The oxidation stabilization treatment of the fibers is carried out to the extent such that the entire polymeric composition of the fibers, when viewed in cross-section, is oxidized. Although the stabilization process, to some extent, depends on the diameter of the fibers, the composition of the polymeric precursor composition, the level of oxygen in the atmosphere, and the treatment temperature, the process is extremely time consuming and costly in order to achieve complete stabilization of the fibers throughout their cross section.
Traditionally, the stabilization treatment of polymeric fibers under oxygen extends over at least several hours to in excess of 24 hours to completely permeate the fibers with oxygen and to achieve sufficient stabilization of the fibers in preparation for subsequent carbonization of the stabilized fibers to produce carbonaceous fibers for commercial end uses. The Encyclopedia of Polymer Science and Engineering, Vol. 2, A Wiley-Interscience Publication, 1985, pp. 641-659, reports that "current standard processing technology requires from 1 to 2 hours for adequate stabilization" of fibers, p.658. No other method of processing suitable for large or "heavy" 320 k tows is disclosed. Also, in "High Performance Fibers II, published by Battelle, esp. the chapter entitled "Process Technology-Oxidation/Stabilization", page 149 et seq. it is reported that oxidation and cyclization takes place between 150.degree. C.-300.degree. C. and that "the reaction must take place throughout the fiber and not be confined to the fiber surface." Accordingly, the lengthy stabilization treatment employed in present standard procedures reduces the productive output of stabilized fibers, requires substantial capital investment, and is therefor extremely costly and a major deterrent in rendering the process desirable for greater commercial exploitation, i.e. extended commercial usage of the fibers at lower cost. It is also reported that if electrically heated oxidation chambers are used, the chambers must be substantially larger than the ovens used in a subsequent carbonization step, therefore resulting in a substantially higher capital cost.
It is further taught in U.S. Pat. No. 4,837,076 that the conventionally stabilized fibers (stabilized precursor fibers) are subsequently formed into a coil-like and/or sinusoidal shape by knitting or weaving a fiber tow into a fabric or cloth. The so formed knitted fabric is thereafter heat treated in a relaxed and unstressed condition and in a non-oxidizing atmosphere at a temperature of from 525.degree. C. to 750.degree. C. and for a period of time sufficient to produce a heat induced thermoset reaction wherein additional crosslinking and/or cross chain cyclization occurs between the original polymer chains. The carbonization treatment of the fibers is carried out to the extent such that the entire oxidation stabilized material of the precursor fibers, when viewed in cross-section, is carbonized. Specifically, no residual portion of the oxidation stabilized fiber material remains in a thermoplastic condition. In example 1 of U.S. Pat. No. 4,837,076, it is reported that portions of a stabilized knitted cloth were heat set at temperatures ranging from 550.degree. C. to 950.degree. C. over a 6 hour period. The most flexible fibers and fibers that are subject to the least fiber breakage due to brittleness when subjected to textile processing were obtained in those fibers that had been heat treated at a temperature of from 525.degree. C. to 750.degree. C. The resulting fiber tows, obtained by deknitting the cloth, and having the heat set, i.e. thermoset, non-linear structural configuration, can then be subjected to other methods of treatment known in the art to create an opening, a procedure in which a yam or the fiber tows of the cloth are separated into an entangled, wool-like fluffy material, in which the individual fibers retain their coil-like or sinusoidal configuration, yielding a fluff or batting-like body of considerable loft.
U.S. Pat. No. 4,837,076 also discloses that at a treatment temperature above 1000.degree. C. the stabilized precursor fibers become graphitic and highly electrically conductive to the point where they begin to approach the conductivity of a metallic conductor. These graphitic fibers find special utility in the manufacture of electrodes for energy storage devices. Since graphitization of the stabilized fibers is carried out at a temperature and for a period of time such that the entire stabilized polymeric material of the fiber, when viewed in cross-section, is graphitized, the process, especially at the higher temperatures, is extremely time and energy consuming and equipment intensive, and therefor very costly.
Graphitization of oxidation stabilized fibers is generally desired in order to produce higher tensile modulus properties in the fibers. However, it is reported in High Performance Fibers II, published by Battelle, Copyright 1987, esp. the chapter entitled "Process Technology-Graphitization", pages 158 and 159, that "breakage of the fibers is a problem that has not been solved" and that "the most serious disadvantage of these high tensile strength fibers is their low strain-to-failure ratio, which means that they are very brittle". Moreover, the process is also said to be expensive because of the "high capital cost of the equipment and the great amount of electrical energy required to achieve the necessary temperature for graphitization of the fibers (2000.degree. to 3000.degree. C.) throughout their entire cross-section.".
Fibers that are generally referred to as "bicomponent or composite fibers", "biconstituent fibers", "bilateral fibers" and "sheath-core fibers" are commonly known in the art. Definitions of these terms can be found in "Man-Made Fiber and Textile Dictionary", Hoechst Celanese Corporation, 1990, pp. 14, 15, 32, and 139. A bicomponent or composite fiber is defined as a fiber composed of two or more polymer types in a sheath-core or side by side (bilateral) relationship. Biconstituent fibers are defined as fibers that are extruded from a homogeneous mixture of two different polymers wherein such fibers combine the characteristics of the two polymers into a single fiber. Bilateral fibers are two generic fibers or variants of the same generic fiber extruded in a side by relationship. Sheath-core fibers are bicomponent fibers of either two polymer types or two variants of the same polymer. One polymer forms a core and the other polymer of a different composition surrounds it as a sheath.
Bicomponent fibers have also been generally disclosed in U.S. Pat. No. 4,643,931, issued Feb. 17, 1987 to F. P. McCullough et al. These fibers are blends of a small amount of conductive fibers into a yarn to act as an electrostatic dissipation element. Fiber manufacturers also routinely manufacture conductive fibers by incorporating into a hollow fiber a core of carbon or graphite containing thermoplastic composite or by coating a fiber with a sheath made of a thermoplastic composite containing carbon or graphite.
U.S. Pat. No. 5,260,124, issued Nov. 9, 1993 to J. R. Gaier, discloses a hybrid material comprising a fabric of high strength carbon or graphite fibers, a layer of a graphitized carbon disposed on the fibers, and an intercalate in the layer. In the process of manufacture, Gaier's fabric of high strength carbon or graphitic fibers is coated by vapor deposition with a porous graphite layer to form a two-dimensional fabric like structure. In contrast to Gaier, the fibers of the invention are "biregional" and are not carbonized or graphitized throughout to form a high strength fiber, nor are the ignition resistant biregional fibers of the invention coated with a layer of graphitized carbon, thereby forming a composite structure. The core region of the fiber of the invention always remains thermoplastic, while the sheath region of the fiber is oxidation stabilized and thermoplastic, or carbonaceous and thermoset Moreover, the ignition resistant biregional fiber of the invention does not require an intercalate treatment in the outer graphite layer.
Electrical energy storage devices, particularly batteries, which employ fibrous carbon or graphite electrodes and which operate in a nonaqueous electrolyte at ambient temperature are known from U.S. Pat. No. 4,865,931, issued Sep. 12, 1989 to F. P. McCullough et al. The patent generally discloses a secondary battery comprising a housing having at least one cell positioned in the housing, each cell comprising a pair of electrodes made of a multiplicity of electrically conductive carbon fibers, a foraminous electrode separator for electrically insulating the electrodes from contact with each other, and an electrolyte comprising an ionizable salt in a nonaqueous fluid in each cell.
A similar electrical storage device is disclosed in U.S. Pat. No. 4,830,938 to F. P. McCullough et al, issued May 16, 1989. This patent discloses a shared bipolar, carbonaceous fibrous, electrode which is capable of carrying a current from one cell to an adjacent cell without a current collector frame associated therewith. Neither of the aforementioned McCullough et al patents disclose the use of ignition resistant biregional fibers having an inner core region of a thermoplastic polymeric composition and a surrounding outer sheath region of a thermoset carbonaceous material. The biregional fibers of the invention are particularly suitable for use as electrodes in secondary energy storage devices primarily in view of their substantially greater flexibility and lower cost.
In general, the biregional fibers of the invention distinguish over the various types of fibers of the prior art in that the biregional fiber is preferably produced from a homogenous polymeric composition, i.e. a single polymeric composition, preferably an acrylic polymer, in which an outer region of the fiber is oxidation stabilized and then carbonized to form two visually distinct regions in the fiber, when viewed in cross section, i.e. typically a translucent or lightly colored inner core region and a black outer sheath region. In the case of a biregional precursor fiber, the fiber comprises a thermoplastic inner core and a thermoplastic stabilized outer sheath, while in the case of an ignition resistant biregional fiber, the inner core is thermoplastic and the outer sheath is thermoset and carbonized.
When the ignition resistant biregional fiber of the invention is manufactured from a homogeneous polymeric composition, preferably an acrylic polymer, there is no boundary or discontinuity between the inner core and the outer oxidation stabilized or carbonized sheath. The term "homogeneous polymeric composition" used herein includes homopolymers, copolymers and terpolymer and does not include fibers containing two or more polymers of different compositions and coefficients of crystallinity. In contrast discontinuities are produced in bilayered or bicomponent fibers in which two polymers of different compositions are used in a side by side or core-sheath relationship. Such discontinuities or boundaries occur between the layers of the different polymeric compositions due to their different coefficients of crystallinity. This also applies to different polymeric compositions which are intermixed with each other.
In the case of a core/sheath fiber, the outer sheath layer is formed much like a skin layer and is separate and distinct from the inner core thus forming a physical boundary or discontinuity between the inner core and the outer skin layer. More specifically, in viewing a cross sectional surface of a bilayered or sheath-core fiber (generally coextruded), inspection of the surface from an outer periphery to the center of the fiber surface, one would pass from one type of polymeric composition forming the outer sheath layer through a boundary layer or discontinuity into the core having another polymeric composition of different crystallinity. As previously indicated, polymers having different compositions also have different coefficients of crystallinity and melting points. For example, polyacrylonitrile will undergo a melting point transition at a temperature range of 320.degree. C.-330.degree. C. This represents a relatively high melting point for polymers and is characteristic of stiff chains. Both nylon 6,6 and PET fibers melt at 265.degree. C., and polyolefins such as polyethylene and polypropylene melt around 135.degree. C. and 165.degree. C., respectively. Accordingly, although the inner core and the outer sheath of the biregional fiber of the invention forms two visually distinct regions, when viewed in cross section, they do not form a physical boundary or discontinuity between the core and the sheath, i.e. the regions are continuous.
The single homogenous polymeric composition that is preferably employed in the manufacture of the ignition resistant biregional fiber of the invention is a standard acrylic polymer, i.e. homopolymer, copolymers and terpolymers of acrylonitrile, wherein the copolymers and terpolymers contain at least 85 mole percent acrylic units and up to 15 mole percent of one or more vinyl monomers copolymerized therewith, or optionally, a subacrylic polymer, as hereinafter disclosed.